In principle, a portable defibrillator consists of a battery, a DC / DC converter, a capacitor, an output circuit and a control unit. Since the voltage of the battery for an electric shock is too small to be fitted with the aid of a DC / DC converter generates portable defibrillator a higher voltage, the capacitor is charged to a previously set energy. By pressing a button gives the capacitor's stored energy, about 200 to 360 joules, from the patient. The voltage is 750 volts and is located at 1 to 20 milliseconds. The current is reached, assuming an average body resistance is 50 ohms up to about 15 amperes. The necessary is capacitor has a capacity of about 1,000 nF.

This energy is delivered through large electrodes, which are either pressed with the hands on the chest of the patient (the so-called "paddles") or be glued to the chest ("adhesive electrodes" or "fast patch"). Especially for publicly accessible portable defibrillators ("PAD" public access defibrillator) - are to simplify operation and reduce the risk of electric shock to the user - virtually only used adhesive electrodes.
For use with open chest (during surgery), there are versions with spoon electrodes which are applied directly to the heart muscle.

The output circuit provides for the portable generation of certain pulse shapes. The controller controls the charging of the capacitor, and directs the output circuit also ensures that is discharged in the absence portable defibrillator of shock delivery, the capacitor has an internal resistance (circuit protection).

Modern biphasic defibrillators work. This means that is emitted from the output stage is not only a shock, but that will be delivered by voltage transitions at the paddles and shocks in the opposite direction. Modern portable defibrillators measure before the release of energy the body resistance (impedance) of the patient by means of electrodes glued and adjust current and voltage is applied to this resistance. Slim, small patients with low impedance receive less power than, for example, obese, large patients. In international guidelines for defibrillation is now specified that portable defibrillators are to deliver an equal height or higher energy than monophasic devices. There is now also an evidence that escalating energy levels are more effective than the same energy taxes to the patient.